About 16,000 years ago (long before the night elves foolishly called down the wrath of the Burning Legion), trolls lorded over much of Kalimdor (then a single continent). There were twin troll empires -- the Gurubashi Empire of the southeastern jungles -- and the Amani Empire of the middle forestlands.
There were smaller tribes that lived far to the north (in the region now known as Northrend). These tribes founded a small nation known as Gundrak, but never achieved the size or prosperity of the southern empires.
The Gurubashi and Amani empires had little love for one another, but rarely warred against each other. At the time, their greatest common enemy was a third empire -- the civilization of Azj'Aqir. The aqir were intelligent insectoids who ruled the lands of the far west. These clever insectoids were greatly expansionistic and incredibly evil. The aqir were obsessed with eradicating all non-insect life from the fields of Kalimdor.
The trolls fought them for many thousands of years, but never succeeded in winning a true victory over the aqir. Eventually, due to the troll's persistence, the aqiri kingdom split in half as its citizens fled to separate colonies in the far northern and southern regions of the continent.
Two aqiri city-states emerged -- Azjol-Nerub in the northern wastes, and Ahn'Qiraj in the southern desert. Though the trolls suspected that there were other aqiri colonies beneath Kalimdor, their existence was never verified.
With the insectoids driven into exile, the twin troll empires returned to business as usual. Despite their great victory, neither civilization expanded much farther than their original boundaries. However, ancient texts speak of a small faction of trolls that broke off from the Amani Empire and founded their own colony in the heart of the dark continent.
There, these brave pioneers discovered the cosmic Well of Eternity which transformed them into beings of immense power. Some legends suggest that these adventurous trolls were the first night elves, though this theory has never been proven.